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INTRODUCTION Environmental Setting Legal agreements in 1928 (Carlson and Carlson 1982) separated the Colorado River into upper and lower basins at Lee Ferry, Arizona. The upper basin was subsequently sub-divided into three hydrologic sub-basins: Green, Upper Main-Stem, and San Juan-Colorado (Iornes et ~l. 1965). The larg~st sub- basin, the Green River, comprises about 116,000 km of the 284,000 km occupied by the upper Colorado River basin (see Preface for English conversion factors). The Green River extends about 800 km to its source, the Wind River Range, in western Wyoming. The study area included the Green River from its confluence with the Colorado River (km 0) to its confluence with the Yampa River (km 552} in Dinosaur National Monument (DNM; Figure 1). The Yampa River was included because the movements and distribution of the target fishes included both rivers, and Yampa River flows affect habitats of fishes in the Green River. The Green River above its confluence with the Yampa River has been influenced by operations of Flaming Gorge Dam since its completion and filling in 1962. Endangered fishes still inhabit the Echo Park area of the Green River and are incidental from Lodore Canyon to Swallow Canyon. However, native fish habitat in the Green River upstream of DNM has been greatly altered by the Dam (Pearson 1967; Vanicek 1967) and no longer supports endangered fish populations. The Yampa and Green rivers contain relatively high-gradient reaches dominated by boulder, cobble, and gravel substrates (e.g., Cross Mountain and Yampa canyons, Yampa River; Whirlpool, Split Mountain, Desolation and Gray canyons, Green River), and low-gradient alluvial reaches of meandering canyon and flat, open terrains dominated by finer substrates. The Yampa River, with an average annual discharge of about 1.5 million-acre-feet, arises on the western slope of the Rocky Mountains near Steamboat Springs, Colorado, then flows northward and westward, and enters DNM and Yampa Canyon below its confluence with the Little Snake River. Upper Yampa Canyon (km 72-32) has a moderate gradient and is characterized by rocky runs, shallow riffles, and rapids. The lower Yampa River deepens, widens, and becomes slower moving as it passes to its confluence with the Green River at Echo Park. The Yampa River flows about 320 km and drops in elevation about 2,256 m (Joseph et al. 1977). Downstream of its confluence with the Yampa River, the Green River enters upper Whirlpool Canyon, a reach dominated by deep eddies, riffles, sheer cliffs, and boulder-strewn banks. The Green River then meanders through sand and cobble shorelines in Island and Rainbow parks where cottonwood, willow, and saltcedar grow along the river bank. The river passes through Split Mountain Canyon, a moderately high-gradient reach dominated by riffles, runs, and rapids, and enters the Uintah Basin where it again becomes broad and shallow. The Duchesne River, draining the southern slope of the Uintah Mountains, enters the Green River near Ouray, Utah (km 396.48). The White River, with headwaters on the western slope of the Rocky Mountains, enters the Green River just below the mouth of the Duchesne River. Below these large 4